House Spider (Tegenaria gigantea) | Credit: Talita Bateman — House Spider (*Tegenaria gigantea) |* Credit: Talita Bateman

Spiders are amazing creatures and there are thousands of identified species in the world. In the UK alone, there are over 600 species across 37 families. Many people will commonly refer to spiders as 'insects’. Although spiders, like insects, are invertebrates and belong to the phylum Arthropoda, they do not belong to the same class. Insects are part of the Insecta class, whereas spiders - similarly to scorpions, mites and ticks - belong to the class Arachnida.

There are many differences between insects and arachnids, mostly comprised of their body structure. In spiders, however, even the life cycle is different. Spiders will hatch from the eggs as little miniature versions of mature spiders, rather than in a larva or nymph form, and their maturing is often observed in their frequent moults.

If you live in the UK, you are likely to have encountered this beauty inside your house or in a shed before. The Tegenaria genus includes a few species that are fairly similar and you may even need a microscope to be able to tell them apart. I found the specimen displayed in the image above (T. gigantea) in my house near Cambridge. It seemed to have, unfortunately, drowned in the sink. I only had a mini microscope to hand (250x) but I was still able to take a couple of photographs that helped me identify the species and its gender.

Deceased House Spider (Tegenaria gigantea) measurements | Credit: Talita Bateman — Deceased House Spider (*Tegenaria gigantea*) measurements | Credit: Talita Bateman

The anatomy of spiders is fascinating and very alien to most of us - I mean, not many creatures have six or even eight eyes! The photographs below give us a dorsal view (Figure 1) and ventral view (Figure 2) of the house spider specimen (T. gigantea) I collected. Although the photos don’t label them, it is clear to see that the spider has, as they should, eight legs - we’ll go into a bit more detail of the anatomy of their leg later on.

Figure 1 - Spider Anatomy: Dorsal view of a Tegenaria gigantea | Credit: Talita Bateman — Figure 1 - Spider Anatomy: Dorsal view of a *Tegenaria gigantea* | Credit: Talita Bateman

Figure 2 - Spider Anatomy: Venral view of Tegenaria gigantea | Credit: Talita Bateman — Figure 2 - Spider Anatomy: Venral view of *Tegenaria gigantea* | Credit: Talita Bateman

From the ventral view (Figure 2), we can determine that this specimen is a mature female. This is because the epigyne is clearly present. While males lack an epigyne completely, it is only visible in females that have gone through their final moult. The epigyne (or epigynium), as you may have guessed, is the external genital structure of a female spider. However, they can greatly vary in appearance in different species and are often used as a method of species identification in some spiders. In fact, their difference in structure is what, most times, prevents inter-species mating.

Figure 3 gives us a close-up view of a mature female’s epigyne but it also gives us a better view of its book lungs, epigastric furrow and pedicel (refer to Figure 2). The book lungs, as the name suggests, is the air-filled cavity through which a spider breathes. The name ‘book lung’ derives from its internal shape - thin vascular sheets arranged like book pages. We can clearly see the epigastric furrow, which is the fold that separates the book lungs and epigyne from the lower part of the spider’s underside abdomen. We can also see the pedicel, the narrow stalk that connects the spider’s cephalothorax and the abdomen.

Figure 4 shows us a very interesting part of the spider’s anatomy - the spinnerets. These are sometimes referred to as ‘spinners’ and are connected to the spider’s underlying silk glands. Through them, silk strands emerge and the spider is able to produce their amazing webs. In the case of the T. gigantea, the web is the classical ‘cobweb’ - a sheet web extending from a tubular retreat.

In Figure 5, we can more closely see the mouth, the fangs and the chelicerae (jaws). We can also see, from a ventral view, the beginning of its legs, starting with the coxa (Brazilians our there, rejoice) and the trochanter. Figure 6 gives us the continuation of the leg from a dorsal view, with some familiar names - femur, patella and tibia followed by the metatarsus and tarsus.

In Figure 7, we can more clearly see the ocular region but, unfortunately, not all the eyes. The ocular region can once again greatly vary in different species. British spiders have 6 or 8 eyes but other species can have fewer (always in even numbers) and some cave-dwelling spiders can have no eyes at all, as it’s been recorded by Dr Peter Jager in his 2012 paper (see reference list for paper).

Much like amphibians and reptiles, spiders are subjected to society’s fear of the unknown - and let’s face it, we humans do not deal well with fear. We have given it a good shot at decimating anything that is perceived as a threat. A spider’s ‘alien’ appearance and fictional and historical link with ‘evil’ make them somehow, erroneously, less worthy of people’s respect (for some). We forget how important these creatures are to the environment and how we are all interdependent. Having spiders in your house can be a great way of naturally controlling unwanted pests such as earwigs, mosquitoes, flies clothes moths, etc. and education is a key factor. Let’s all be more open to learning about these creatures and admiring and respecting them for what they are - amazing and beautiful.

References:

Beccaloni, J. (2009). Arachnids. Natural History Museum, London, UK.
Bee, L., Oxford, G. and Smith, H. (2017). Britain’s Spiders: A Field Guide. Princeton University Press, Oxford, UK.
Cowels, J. (2018). Amazing Arachnids. Princeton University Press, Oxford, UK.
Jager, P. (2012). Revision of the genus Sinopoda Jager, 1999 in Laos with discovery of the first eyeless huntsman spider species (Sparassidae: Heteropodinae). Zootaxa, 3415, 37-57.